1. Field of the Present Invention
The present invention generally relates to the field of data acquisition systems and more particularly to a wireline logging system and method that includes dynamic control of the power supplied to a load based upon a model of the wireline system.
2. History of Related Art
Wireline logging refers generally to the surveying of oil or gas wells to determine their geological, petro-physical, or geophysical properties using electronic measuring instruments. The electronic instruments are conveyed into a wellbore with an armored steel cable, referred to as a wireline cable. Measurements made by downhole instruments secured to the wireline cable are transmitted back to a data processing system located at the surface through electrical conductors in the wireline cable. Electrical, acoustical, nuclear and imaging tools are used to stimulate the formations and fluids within the wellbore and the electronic measuring instruments then measure the response of the formations and fluids. The wireline cable provides also the electrical power required by the logging tools to operate.
From an electric power perspective, a wireline logging system may be viewed as an electrical circuit comprising a head resistance (Rh) representing the downhole instruments in series with an impedance element representing the wireline cable itself. If the wireline cable is modeled as a simple resistive element, FIG. 1 illustrates a circuit diagram of the system. In this simple model, a voltage source Vs is applied across the series combination of the cable resistance (Rc) and the head resistance Rh.
Two competing considerations govern wireline logging operations. On the one hand, it is desirable to maximize the power delivered to the head (tools) to maximize the rate of data acquisition and speed up operations in general. It is well known that maximum power is delivered to the load when the cable resistance Rc equals the head resistance Rh and the voltage at the head Vh is xc2xd the source voltage Vs. Under these conditions, however, the load voltage Vh varies by approximately 100% when the load impedance (Rh) goes from this minimum value to almost open circuit, which occurs when heavy loads are disconnected and only the control circuits remain powered. This variation is typically unacceptable for electronic circuits in general, and in wireline logging systems in particular. Some means of regulating the voltage supplied to the head is therefore required.
Referring now to FIG. 2, a block diagram representing a traditional voltage regulation system 120 is illustrated. Voltage regulation system 120 compares a measured value of the load voltage VL 122 against a set voltage Vset 124 to determine an error signal 126. Error signal 126 is indicative of the difference between VL 122 and the Vset 124. Error signal 126 is then typically provided to an error amplification element 128. Amplification element 128 typically includes an operational amplifier and may employ a proportional, integrating, and/or differentiating circuit depending upon the application. Amplification element 128 typically generates a source voltage VS from error signal 126 to maintain VL at the desired level (i.e., Vset).
A conventional voltage regulator such as system 120 requires an accurate measurement of the load voltage VL to implement the voltage control at the voltage source. In many applications where the load is located in proximity to the source voltage, feeding the load voltage back to the regulator circuitry presents no significant problem. In a wireline logging application, unfortunately, it is logistically impracticable to feed the load voltage back to the source because of the tremendous length of contemporary wireline cables. Even if the wireline cable were constructed to include a feedback cable to carry the load voltage signal back to the surface, the loss and delay that would characterize the feedback cable would result in a significantly degraded load signal. Accordingly, it would be highly desirable to implement a voltage control system that could be suitably employed in a wireline logging application and other applications characterized by great distances between the load and the voltage regulator. This system would preferably allow the operation of the cable at or near its maximum power carrying capacity, while minimizing the voltage variation at the input of the electronic circuits powered. It would be further desirable if the implemented solution did not substantially increase the cost or complexity of the wireline logging operation.